Light scanning apparatus

ABSTRACT

A light scanning apparatus, including: a light source; a deflector having a rotary polygon mirror configured to deflect the light beam emitted from the light source, and a motor configured to rotate the polygon mirror; a plurality of reflecting mirrors configured to reflect the light beam to the photosensitive member; and an optical box on which the light source is mounted, wherein the optical box has an installation wall on which the deflector is installed and a support wall positioned on a side of the photosensitive member with respect to the polygon mirror, the support wall being provided with a support portion configured to support at least one reflecting mirror, a stepped portion having a plurality of steps is formed between the installation wall and the support wall, and a back surface of the stepped portion has a shape following an inside surface of the stepped portion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light scanning apparatus used in animage forming apparatus such as a copying machine, a printer, afacsimile machine, and a multifunctional peripheral thereof.

2. Description of the Related Art

As a light scanning apparatus used in an electrophotographic imageforming apparatus, a light scanning apparatus having the followingconfiguration is well known. Specifically, there has been known a lightscanning apparatus configured to deflect a light beam emitted from alight source by a rotary polygon mirror and to guide the deflected lightbeam onto a photosensitive surface of the photosensitive member byoptical components such as a lens and a mirror, to form a latent imageon a photosensitive member. An imaging optical system is formed of atleast one fθ lens. The fθ lens has a special lens effective surfacetypified by an aspheric surface for the purpose of enhancing scanningcharacteristics. Further, a housing (hereinafter referred to as “opticalbox”) configured to support and fix members of an optical system isformed of a resin molding because of the advantages such as thesecurement of a degree of freedom of a shape, the reduction in weight,and the reduction in price. In particular, the use of an optical boxmade of a resin is greatly advantageous in a tandem type image formingapparatus, because a large number of optical components are used, andfurther mounting directions and mounting methods of supporting andfixing the optical components are not uniform. On the other hand, theoptical box made of a resin has a large coefficient of thermal expansionunder the condition of an increased temperature, compared to an opticalbox made of a metal. Further, the optical box made of a resin has a lowcoefficient of thermal conductivity, compared to the optical box made ofa metal. Therefore, when the optical box made of a resin is used, in thelight scanning apparatus containing a heat source, a temperaturedistribution becomes non-uniform, and a hot portion and a cold portionoccur partially. As a result, warpage and local distortion in differentdirections occur in the optical box.

A deflection member such as a rotary polygon mirror having a pluralityof deflection reflecting surfaces has been often used in the lightscanning apparatus. When the deflection member is driven, thetemperature increases by the heat generation from a driving portion suchas a motor. If light scanning is performed continuously for a longperiod of time, a rotation shaft receiving portion of the rotary polygonmirror and an IC chip mounted on a motor portion configured to drive therotary polygon mirror are heated to a high temperature. Further, even inthe case where light scanning is performed for a short period of time,the temperature logarithmically fluctuates and increases immediatelyafter the start of the rotation of the motor. Therefore, the optical boxis greatly distorted and deformed by a non-uniform increase intemperature in the light scanning apparatus. A lens, the rotary polygonmirror, and a mirror constituting the light scanning apparatus arehoused in the optical box, and hence the positions of optical componentssuch as the lens and the mirror are changed by the deformation of theoptical box, with the result that the path through which a light beampasses and a reflecting direction are changed with time.

The deformation of the optical box occurs mainly for the following twocauses. The first cause is a hot current of air, and the second cause isa radiation heat. The hot current of air which is the first cause occursas follows. The rotary polygon mirror rotates at a high speed and hencegenerates wind, which absorbs heat generated on the periphery of adeflector to become hot current of air. The periphery of the deflectoris surrounded by a rib provided perpendicularly to a bottom wall of theoptical box so as to keep the strength of the optical box, and the wayof the hot current is blocked by the rib so that the hot current of airstagnates around the deflector. Because of this, an increase intemperature on the periphery of the deflector becomes larger than thatin the other portions of the optical box. Consequently, the optical boxincreases in temperature locally, which causes the deformation such aswarpage and distortion in the optical box. Further, the radiation heatwhich is the second cause occurs as follows. The rotary polygon mirrorrotates at a high speed, and hence the rotation shaft receiving portionof the rotary polygon mirror fitted in the optical box so as to positionthe deflector reaches a high temperature. Further, the IC chip mountedon the motor portion also generates heat. Then, portions immediatelybelow the rotation shaft receiving portion and the location where the ICchip is mounted are heated locally by the radiation heat, with theresult that the optical box expands and deforms.

As described above, by the deformation of the optical box, directionsand amounts of light beams vary in different color stations so thatlight-condensing positions on surfaces to be scanned are changed, andthus a horizontal direction, a vertical direction, or a magnification ofan image line is fluctuated, resulting in the degradation in image. Inparticular, in a tandem type image forming apparatus, a light beamposition of each color is fluctuated, and hence color misregistrationoccurs when toner images of respective colors are superimposed. In orderto solve the foregoing problem, there has been proposed a light scanningapparatus in which a rib, which has provided perpendicularly to thebottom portion of a housing main body, is inclined so that the hotcurrent of air generated from a rotary polygon mirror and so on isdiffused along the inclined rib (Japanese Patent No. 4170736).

However, according to the conventional method, certain effects areobtained with respect to the first cause of the deformation of theoptical box described above, but there is a risk in that effects may notbe obtained with respect to the second cause. That is, hot current ofair generated from the deflector can be diffused to the periphery;however, hot current of air is blown to the inclined portion of theoptical box, with the result that the entire optical box is greatlydistorted by the distortion caused by the expansion and deformation ofthe inclined portion. The inclined portion of the optical box has astraight line shape, and hence the expansion thereof serves to distortthe entire optical box. Further, the local deformation of the rotationshaft receiving portion of the rotary polygon mirror and also thedeformation of expansion from the center of the rotation shaft receivingportion to the periphery thereof cannot be absorbed by the inclinedportion formed into a straight line shape, which leads to thedeformation of the entire optical box.

SUMMARY OF THE INVENTION

The present invention has been achieved under the above-mentionedcircumstances, and it is an object of the present invention to reducethe deformation of an optical box with a simple configuration.

In order to solve the above-mentioned problems, according to anembodiment of the present invention, there is provided a light scanningapparatus, comprising: a light source configured to emit a light beam; adeflector including a rotary polygon mirror configured to deflect thelight beam so that the light beam scans a photosensitive member, a motorconfigured to rotate the rotary polygon mirror, a driving unitconfigured to drive the motor, and a circuit board on which the motorand the driving unit are mounted; a plurality of reflecting mirrorsconfigured to reflect the light beam deflected by the deflector to guidethe light beam deflected by the deflector onto the photosensitivemember; and an optical box on which the light source is mounted, theoptical box being configured to contain the deflector and the pluralityof reflecting mirrors, wherein the optical box includes an installationwall on which the deflector is installed and a support wall positionedon a side of the photosensitive member with respect to the rotarypolygon mirror, the support wall being provided with a support portionconfigured to support at least one of the plurality of reflectingmirrors, a stepped portion including a plurality of steps is formedbetween the installation wall and the support wall, and a back surfaceof the stepped portion has a shape following a shape of the steppedportion inside the optical box.

According to another embodiment of the present invention, there isprovided a light scanning apparatus, comprising: a light sourceconfigured to emit a light beam; a deflector including a rotary polygonmirror configured to deflect the light beam so that the light beam scansa photosensitive member, a motor configured to rotate the rotary polygonmirror, a driving unit configured to drive the motor, and a circuitboard on which the motor and the driving unit are mounted; a pluralityof reflecting mirrors configured to reflect the light beam deflected bythe deflector to guide the light beam deflected by the deflector ontothe photosensitive member; and an optical box on which the light sourceis mounted, the optical box being configured to contain the deflectorand the plurality of reflecting mirrors, wherein the optical boxincludes an installation wall on which the deflector is installed and asupport wall positioned on a side of the photosensitive member withrespect to the rotary polygon mirror, the support wall being providedwith a support portion configured to support at least one of theplurality of reflecting mirrors, a stepped portion including a pluralityof steps is formed between the installation wall and the support wall,and a thickness of the stepped portion is smaller than a thickness ofthe installation wall.

According to further another embodiment of the present invention, thereis provided a light scanning apparatus, comprising: a light sourceconfigured to emit a light beam; a deflector including a rotary polygonmirror configured to deflect the light beam so that the light beam scansa photosensitive member, a motor configured to rotate the rotary polygonmirror, a driving unit configured to drive the motor, and a circuitboard on which the motor and the driving unit are mounted; an opticalmember configured to guide the light beam deflected by the rotarypolygon mirror onto the photosensitive member; and an optical box onwhich the light source is mounted, the optical box being configured tocontain the deflector and the optical member, wherein the optical boxincludes an installation wall on which the deflector is installed, asupport wall positioned on a side of the photosensitive member withrespect to the rotary polygon mirror, the support wall being providedwith a support portion configured to support the optical member, and astepped portion including at least two steps in a vicinity of thedeflector.

According to further another embodiment of the present invention, thereis provided a light scanning apparatus, comprising: a light sourceconfigured to emit a light beam; a deflector including a rotary polygonmirror configured to deflect the light beam so that the light beam scansa photosensitive member, a motor configured to rotate the rotary polygonmirror, a driving unit configured to drive the motor, and a circuitboard on which the motor and the driving unit are mounted; a pluralityof reflecting mirrors configured to reflect the light beam deflected bythe deflector to guide the light beam deflected by the deflector ontothe photosensitive member; and an optical box configured to contain thedeflector and the plurality of reflecting mirrors, wherein the opticalbox includes an installation wall on which the deflector is installedand a support wall positioned on a side of the photosensitive memberwith respect to the rotary polygon mirror, the support wall beingprovided with a support portion configured to support at least one ofthe plurality of reflecting mirrors, and a waveform portion having awaveform in cross-section in a direction directed from the installationwall toward the support wall is formed between the installation wall andthe support wall.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a sectional view of an image forming apparatus according to afirst embodiment.

FIG. 1B is a sectional view of a light scanning apparatus according tothe first embodiment.

FIG. 2 is a view illustrating a configuration of the light scanningapparatus according to the first embodiment.

FIGS. 3A, 3B, and 3C are views illustrating a configuration of a steppedportion of the light scanning apparatus according to the firstembodiment.

FIG. 4A is a view illustrating a flow of hot current of air in the firstembodiment.

FIG. 4B is a view illustrating a bent portion of the stepped portion inthe first embodiment.

FIG. 5 is a view illustrating the case where a stepped portion has onestep for comparison with the first embodiment.

FIGS. 6A, 6B, and 6C are views illustrating comparison results betweenthe first embodiment and a conventional example.

FIG. 7 is a view illustrating a configuration of a stepped portion of alight scanning apparatus according to a second embodiment.

DESCRIPTION OF THE EMBODIMENTS

Now, exemplary embodiments of the present invention will be described indetail with reference to the drawings.

First Embodiment Configuration of Image Forming Apparatus

The configuration of an image forming apparatus according to a firstembodiment will be described. FIG. 1A is a schematic structural viewillustrating an entire configuration of a tandem type color laser beamprinter of the embodiment. The laser beam printer (hereinafter referredto simply as “printer”) includes four image forming engines (imageforming portions) 10Y, 10M, 10C, and 10Bk (indicated by dashed-dottedlines) configured to form toner images of respective colors: yellow (Y),magenta (M), cyan (C), and black (Bk). Further, the printer includes anintermediate transfer belt 20 onto which a toner image is transferredfrom each of the image forming engines 10Y, 10M, 10C, 10Bk. Then, theprinter is configured in such a manner that the toner imagesmulti-transferred onto the intermediate transfer belt 20 are transferredonto a recording sheet P serving as a recording medium to form afull-color image.

The intermediate transfer belt 20 is formed into an endless shape and ispassed over a pair of belt conveyance rollers 21, 22 so that a tonerimage formed by each image forming engine 10 is transferred onto theintermediate transfer belt 20 while the intermediate transfer belt 20 isrotated in a direction indicated by an arrow B. A secondary transferroller 65 is provided at a position opposed to one belt conveyanceroller 21 across the intermediate transfer belt 20. The recording sheetP is inserted between the secondary transfer roller 65 and theintermediate transfer belt 20 which are in pressure contact with eachother, with the result that a toner image is transferred from theintermediate transfer belt 20 to the recording sheet P. The four imageforming engines 10Y, 10M, 10C, 10Bk described above are arranged side byside under the intermediate transfer belt 20 so that a toner imageformed in accordance with image information of each color is transferredonto the intermediate transfer belt 20 (hereinafter referred to as“primary transfer”). The four image forming engines 10 are arranged inthe following order: the image forming engine 10Y for yellow, the imageforming engine 10M for magenta, the image forming engine 10C for cyan,and the image forming engine for black 10B in a rotation direction(indicated by the arrow B) of the intermediate transfer belt 20.

Further, a light scanning apparatus 40 configured to expose aphotosensitive drum 50 serving as a photosensitive member provided ineach image forming engine to light in accordance with image informationis provided below the image forming engine 10. Note that, the detailedillustration and description of the light scanning apparatus 40 areomitted in FIG. 1B and described later with reference to FIG. 2. Asillustrated in FIG. 1B, the light scanning apparatus 40 is shared by allthe image forming engines 10Y, 10M, 10C, 10Bk and includes foursemiconductor lasers (not shown) each configured to emit a light beammodulated in accordance with image information of a corresponding color.Further, the light scanning apparatus 40 includes a deflector 43 whichincludes a rotary polygon mirror 42 which rotates at a high speed todeflect the light beams of 4 optical paths so that the light beams scanthe photosensitive drums 50 in a rotation axis direction (Y-axisdirection) and a motor unit configured to rotate the rotary polygonmirror 42. The deflector 43 includes the rotary polygon mirror 42, amotor configured to rotate the rotary polygon mirror 42, a motor unit 41serving as a driving unit configured to drive the motor, and a circuitboard 64 on which the motor and the motor unit 41 are mounted (see FIG.3A). Each light beam scanned by the deflector 43 travels through apredetermined path while being guided by optical members provided in thelight scanning apparatus 40. Then, each light beam which has travelledthrough the predetermined path passes through each irradiation port (notshown) provided in an upper portion of the light scanning apparatus 40to expose each photosensitive drum 50 of each image forming engine 10 tolight.

Each image forming engine 10 includes the photosensitive drum 50 and acharging roller 12 configured to charge the photosensitive drum 50 to auniform background potential. Further, each image forming engine 10includes a developing device 13 configured to develop an electrostaticlatent image, formed on the photosensitive drum 50 (photosensitivemember) by exposure to the light beam, to form a toner image. Thedeveloping device 13 forms a toner image in accordance with imageinformation of each color on the photosensitive drum 50 serving as aphotosensitive member.

A primary transfer roller (primary transfer member) 15 is provided at aposition opposed to the photosensitive drum 50 of each image formingengine 10 across the intermediate transfer belt 20. When a predeterminedtransfer voltage is applied to the primary transfer roller 15, a tonerimage on the photosensitive drum 50 is transferred onto the intermediatetransfer belt 20.

On the other hand, the recording sheet P is supplied from a feedcassette 2 loaded in a lower portion of a printer housing 1 to an innerportion of the printer, specifically to a secondary transfer position atwhich the intermediate transfer belt 20 and the secondary transferroller (secondary transfer member) 65 are in abutment with each other.In an upper portion of the feed cassette 2, a pickup roller 24configured to pick up the recording sheet P contained in the feedcassette 2 and a feed roller 25 are arranged side by side. A retardroller 26 configured to prevent a double feed of the recording sheet Pis provided at a position opposed to the feed roller 25. A conveyancepath 27 of the recording sheet P inside the printer is providedsubstantially vertically along a right side surface of the printerhousing 1. The recording sheet P fed out from the feed cassette 2positioned in a bottom portion of the printer housing 1 ascends throughthe conveyance path 27 and is sent to registration rollers 29 configuredto control a timing of the recording sheet P entering the secondarytransfer position. After a toner image is transferred onto the recordingsheet P at the secondary transfer position, the recording sheet P issent to a fixing device 3 (indicated by a broken line) provided on adownstream side in the conveyance direction. Then, the recording sheet Pon which the toner image has been fixed by the fixing device 3 isdelivered by delivery rollers 28 to a delivery tray 1 a provided in anupper portion of the printer housing 1.

When a full-color image is formed by the color laser beam printerconfigured as described above, first, the light scanning apparatus 40exposes the photosensitive drum 50 of each image forming engine 10 tolight at a predetermined timing in accordance with image information ofeach color. Consequently, a toner image in accordance with the imageinformation is formed on the photosensitive drum 50 of each imageforming engine 10. In order to obtain a good quality image, it isnecessary to reproduce the position of a latent image formed by thelight scanning apparatus 40 with high accuracy.

[Configuration of Light Scanning Apparatus]

FIG. 1B is a schematic view illustrating an overview when the opticalcomponents are mounted, and FIG. 2 is a view illustrating theconfiguration of the light scanning apparatus 40 of the embodiment. Notethat, the light scanning apparatus 40 includes an optical box 105 and acover 70 configured to cover an opening of an upper portion of theoptical box 105. A light source unit 61 on which a light sourceconfigured to emit a light beam is mounted and the deflector 43including the rotary polygon mirror 42 configured to deflect a lightbeam and the motor unit 41 are provided on an outer peripheral portionof and inside the light scanning apparatus 40. Further, the lightscanning apparatus 40 includes an optical lens 60 (60 a to 60 d) and areflecting mirror 62 (62 a to 62 h) configured to guide light beams tothe photosensitive drums 50 to provide images on the photosensitivedrums 50. The optical box 105 includes an installation wall 105 a onwhich the deflector 43 is installed and a support wall 105 b positionedon a side of the photosensitive drum 50 (photosensitive member-side)with respect to the rotary polygon mirror 42, on which a support portionconfigured to support at least one of the reflecting mirrors 62 a to 62h is formed.

A light beam LY corresponding to the photosensitive drum 50Y emittedfrom the light source unit 61 is deflected by the rotary polygon mirror42 and enters the optical lens 60 a. The light beam LY having passedthrough the optical lens 60 a enters the optical lens 60 b, and passesthrough the optical lens 60 b to be reflected by the reflecting mirror62 a. The light beam LY reflected by the reflecting mirror 62 a scansthe photosensitive drum 50Y through a transparent window (not shown).

A light beam LM corresponding to the photosensitive drum 50M emittedfrom the light source unit 61 is deflected by the rotary polygon mirror42 and enters the optical lens 60 a. The light beam LM having passedthrough the optical lens 60 a enters the optical lens 60 b, and passesthrough the optical lens 60 b to be reflected by the reflecting mirrors62 b, 62 c, and 62 d. The light beam LM reflected by the reflectingmirror 62 d scans the photosensitive drum 50M through a transparentwindow (not shown).

A light beam LC corresponding to the photosensitive drum 50C emittedfrom the light source unit 61 is deflected by the rotary polygon mirror42 and enters the optical lens 60 c. The light beam LC having passedthrough the optical lens 60 c enters the optical lens 60 d, and passesthrough the optical lens 60 d to be reflected by the reflecting mirrors62 e, 62 f, and 62 g. The light beam LC reflected by the reflectingmirror 62 g scans the photosensitive drum 50C through a transparentwindow (not shown).

A light beam LBk corresponding to the photosensitive drum 50Bk emittedfrom the light source unit 61 is deflected by the rotary polygon mirror42 and enters the optical lens 60 c. The light beam LBk having passedthrough the optical lens 60 c enters the optical lens 60 d, and passesthrough the optical lens 60 d to be reflected by the reflecting mirror62 h. The light beam LBk reflected by the reflecting mirror 62 h scansthe photosensitive drum 50Bk through a transparent window (not shown).

FIG. 2 illustrates the light scanning apparatus 40 with the cover 70removed so that the inside of the optical box 105 can be seen, andillustrates a configuration of the embodiment for reducing the change inposition of a light beam caused by an increase in temperature. The lightsource unit 61 on which the light source configured to emit light beamsis mounted and the deflector 43 configured to reflect and deflect thelight beams are provided on an outer peripheral portion of and insidethe light scanning apparatus 40. Further, the optical lens 60 (60 a, 60b) and the reflecting mirror 62 required for guiding light beams to thephotosensitive drums 50 to provide images on the photosensitive drums 50are provided in the light scanning apparatus 40. Note that, in thefollowing description, the rotation axis direction of the rotary polygonmirror 42 of the deflector 43 is defined as a Z-axis direction, the mainscanning direction which is a scanning direction of the light beam orthe longitudinal direction of the optical lens 60 or the reflectingmirror 62 is defined as the Y-axis direction, and the directionorthogonal to the Y-axis and the Z-axis is defined as an X-axisdirection.

The light beam deflected and scanned by the deflector 43 passes throughthe first optical lens 60 a having a strong power in the main scanningdirection (Y-axis direction), and is then guided to the second opticallens 60 b having a strong power in a sub-scanning direction (X-axisdirection). The light beam having passed through the first optical lens60 a and the second optical lens 60 b is reflected at least once by thereflecting mirror 62 and guided to the photosensitive drum 50 serving asa surface to be scanned to form an image.

(Stepped Portion)

A stepped portion 68A and a stepped portion 68B are formed so as to beintegrated with the optical box 105 of the embodiment. For example, asillustrated in FIGS. 3A and 3B, the stepped portion 68A and the steppedportion 68B are formed into a stepped shape of a plurality of stepsincluding at least two steps in a manner that a plurality of surfacesare bent and connected (a waveform portion such as an accordion shape ora triangular waveform). In other words, the stepped portion 68A and thestepped portion 68B are formed of at least two steps so that the heightincreases as the stepped portion is distanced from the deflector 43. Thethickness of the stepped portions 68A and 68B is smaller than that ofthe installation wall 105 a on which the deflector 43 is installed. Forexample, the thickness of the stepped portions 68A and 68B is 2.5 mm,and the thickness of the installation wall 105 a is 3 mm. The sameapplies to the case where the stepped portions 68A and 68B are waveformportions. Note that, FIGS. 3A and 3B are sectional views taken along theline IIIA-IIIA of FIG. 2, and the same applies to FIGS. 5 and 7.

Now, one of the plurality of steps forming the stepped portion 68A isfocused. As illustrated in FIG. 3B, one step is formed of a first wall68A1 substantially perpendicular to the installation wall (hereinafterreferred to as “bottom wall”) 105 a, which is parallel to the XY-plane,of the optical box 105 and a second wall 68A2 which is substantiallyperpendicular to the first wall 68A1 and which is substantially parallelto the bottom wall 105 a of the optical box 105. One step comprises thefirst wall 68A1 and the second wall 68A2, and at least two steps arecontinuously provided to form the stepped portion 68A of the embodiment.Similarly, one step comprises a first wall 68B1 and a second wall 68B2,and at least two steps are continuously provided to form the steppedportion 68B. In this case, the first wall 68B1 is substantiallyperpendicular to the bottom wall 105 a, which is parallel to theXY-plane, of the optical box 105, and the second wall 68B2 issubstantially perpendicular to the first wall 68B1 and substantiallyparallel to the bottom wall 105 a of the optical box 105. Further,surfaces 68A_b and 68B_b (which also serve as back surfaces) on an outerside of the stepped portions 68A and 68B (see FIG. 3C) are shaped so asto follow surfaces 68A_f and 68B_f of the stepped portions 68A and 68Bon an inner side of the optical box 105 so that the stepped portions 68Aand 68B can be thermally deformed.

Note that, in the embodiment, the stepped portion 68A serving as a firststepped portion is provided between the deflector 43 and the lightsource unit 61 and is formed of two steps. On the other hand, thestepped portion 68B serving as a second stepped portion is provided onan opposite side of the stepped portion 68A with respect to thedeflector 43 and is formed of four steps. Specifically, the steppedportion 68B is provided between the deflector 43 and an outer wall ofthe optical box 105 on an opposite side of the light source unit 61 withrespect to the deflector 43. Note that, the reason that the number ofsteps of the stepped portion 68A is smaller than that of the steppedportion 68B will be described later.

[Regarding Radiation Heat]

As described above, a deflector bearing portion 69 of the deflector 43generates heat when the rotary polygon mirror 42 of the deflector 43rotates at a high speed. Further, an IC chip 67 mounted on the motorunit 41 configured to drive the rotary polygon mirror 42 of thedeflector 43 also generates heat. Therefore, as illustrated in FIG. 3A,the optical box 105 is locally heated by radiation heat in a bearingportion vicinity N1 where the deflector bearing portion 69 is fittedinto the optical box 105 and in a portion N2 immediately below theposition where the IC chip 67 is mounted. When the deflector bearingportion 69 and the portion of the optical box 105 immediately below theposition where the IC chip 67 is mounted are heated, a stress acts indirections indicated by the arrows L in FIG. 3A by the local expansionof the optical box 105, and the optical box 105 is deformed so as topush away surrounding members. Note that, such a stress is also referredto as an expansion deformation stress.

In the embodiment, by providing the stepped portions 68A and 68B on theoptical box 105, the deformation of the optical box 105 caused by thestress acting in the directions indicated by the arrows L can beabsorbed by the stepped portions 68A and 68B. Specifically, in thestepped portion 68A, the first wall 68A1 forming the step is notsubstantially perpendicular to the bottom wall 105 a of the optical box105 because of the stress acting in the directions indicated by thearrows L, and the second wall 68A2 is not substantially perpendicular tothe first wall 68A1 because of the stress acting in the directionsindicated by the arrows L (see FIG. 3A). On the other hand, similarly inthe stepped portion 68B, the first wall 68B1 forming the step is notsubstantially perpendicular to the bottom wall 105 a of the optical box105 because of the stress acting in the directions indicated by thearrows L, and the second wall 68B2 is not substantially perpendicular tothe first wall 68B1 because of the stress acting in the directionsindicated by the arrows L. Thus, the stepped portions 68A and 68B of theembodiment are deformed by the stress acting in the directions indicatedby the arrows L. Then, the stepped portions 68A and 68B are deformed bythe stress acting in the directions indicated by the arrows L so thatthe entire optical box 105 is prevented from being deformed by thestress acting in the directions indicated by the arrows L. Therefore, itcan be said that the stepped portions 68A and 68B absorb the deformationof the entire optical box 105 caused by the stress acting in thedirections indicated by the arrows L.

In particular, when the attitude of the light source unit 61 provided onthe side of the stepped portion 68A is changed by the deformation of theoptical box 105, the optical characteristics of the light source unit 61is changed. Therefore, the technical effects obtained by causing thestepped portion 68A to absorb the deformation of the optical box 105 aregreat. Accordingly, the deformation of the entire optical box 105 can bereduced by causing the stepped portion 68A to absorb the deformation ofthe optical box 105, with the result that the change in irradiationposition of a light beam emitted from the light source unit 61 can bereduced.

Further, as described above, the stepped portion 68B is provided on anopposite side of the light source unit 61 with respect to the deflector43. In the same way as the stepped portion 68A, the stepped portion 68Bcan also prevent an expansion deformation stress caused by heat of thedeflector bearing portion 69 and the like from being transmitted to thesurrounding more widely. Unlike the outer wall of the optical box 105 onthe side where the light source unit 61 is provided, the outer wall ofthe optical box 105 on an opposite side with respect to the deflector 43is not provided with holes and the like and hence has high stiffness.When this outer wall is increased in temperature and deformed, warpageand distortion of the entire optical box 105 are caused. However, byproviding the stepped portion 68B in a place opposite to the steppedportion 68A with respect to the deflector 43, the entire distortion ofthe optical box 105 can be suppressed, and further the change inirradiation position of a light beam can be reduced.

After the operation of the light scanning apparatus of the image formingapparatus starts, the bearing portion vicinity N1 and the portion N2immediately below the IC chip 67 of the optical box 105 become hottest,and the bearing portion vicinity N1 and the portion N2 immediately belowthe IC chip 67 start to expand largely. The deformation caused by theexpansion of the bearing portion vicinity N1 and the portion N2immediately below the IC chip 67 presses the surrounding optical box'smaterial and spreads to the surrounding, and hence a stress acts in thedirections indicated by the arrows L illustrated in FIG. 3A. Whenreceiving the stress acting in the directions indicated by the arrows L,the stepped portions 68A and 68B are deformed to absorb the stress,thereby preventing other portions of the optical box 105, for example, amirror seating surface which greatly influences a change in irradiationposition of a light beam from being deformed.

[Regarding Hot Current of Air]

As illustrated in FIG. 3B, the stepped portions 68A and 68B of theembodiment are configured so that a side G farther away from thedeflector 43 is positioned higher than a side F closer to the deflector43. In this configuration, when the deflector 43 rotates at a highspeed, an air flowed outwardly from the center of the deflector 43 canbe smoothly guided from the periphery of the deflector 43 to an outerside as indicated by arrows W. The current of air generated by thehigh-speed rotation of the deflector 43 is hot current of air containingheat released from the deflector bearing portion 69 and the IC chip 67.Therefore, in the case where the deflector 43 is surrounded by a rib forreinforcement, the rib being provided perpendicularly to the bottom wall105 a of the optical box 105 as in the conventional art, hot current ofair stagnates in the periphery of the deflector 43 to cause a state inwhich the hot air becomes stagnant without flowing. Therefore, in theconventional configuration, the local deformation of the optical box 105is accelerated.

However, by providing the stepped portions 68A and 68B as in theembodiment, the hot current of air which has occurred can be diffusedsmoothly in a wide range along the slope formed by the stepped portions68A and 68B. This can reduce a difference in temperature occurring inthe light scanning apparatus 40. Note that, there is a gap between thehighest step of the stepped portion 68B and the cover 70 of the opticalbox 105, and hence a flow of hot current of air indicated by the arrowsW can be diffused from the gap to the other portions of the optical box105. Accordingly, the thermal deformation of the optical box 105 causedwhen hot current of air stagnates in the vicinity of the deflector 43can also be reduced, and color misregistration caused by a change inirradiation position of a light beam can be greatly reduced.

(Regarding Rib of Stepped Portion)

The slope of the stepped portion 68B of the embodiment extends frombelow to above so as to intersect with a plane (parallel to theXY-plane) that passes through a deflection point at which a light beamis deflected and that is perpendicular to a deflecting surface (parallelto the Z-axis). With this configuration, hot current of air which hasflowed out by the high-speed rotation of the rotary polygon mirror 42 ofthe deflector 43 is reliably guided and smoothly sent to thesurrounding. Further, the stiffness of the optical box 105 can beenhanced by forming a rib provided in a space R defined by thedifference in height of the steps.

For example, a rib 68C of the stepped portion 68B of the embodiment isprovided as illustrated in FIG. 3C. FIG. 3C is a view of the optical box105 when viewed from the bottom wall 105 a of the optical box 105, thatis, when viewed from the back side of the optical box 105. Further, asillustrated in FIG. 3C, in the space R, the rib 68C extending along theline IIIA-IIIA of FIG. 2 is provided on the optical box 105. In orderfor the stepped portion 68 to absorb the stress acting in the directionsindicated by the arrows L effectively, it is desired that the number ofribs provided in the space R be smaller. On the other hand, in order toenhance the stiffness of the optical box 105, it is desired that thenumber of ribs provided in the space R be larger. Thus, the number ofribs for reinforcement provided in the space R is determined based onthe balance between the effect of the absorption of a stress by thestepped portion 68 and the effect of enhancing stiffness by the rib withrespect to each optical box.

Further, as for the stepped portion 68 of the embodiment, the heightfrom the bottom wall 105 a of the optical box 105 becomes larger in thestepped portion 68B provided on the side opposite to the light sourceunit 61 with respect to the deflector 43, than in the stepped portion68A provided between the deflector 43 and the light source unit 61. Thatis, the height of the stepped portion 68B provided on a side opposite tothe stepped portion 68A is set to be higher than the height of thestepped portion 68A provided on an incident light side of a light beamon which the height of the stepped portion 68A is limited. Consequently,a space (also referred to as “sectional space”) through which a lightbeam does not pass can be used effectively, and the stiffness of theoptical box 105 can be further enhanced.

(Regarding Light-Shielding Wall)

In the embodiment, in order to prevent unintended light (hereinafterreferred to as “flare light”) reflected by each lens surface fromentering other image forming engines to illuminate other photosensitivedrums 50, a light-shielding wall 66 configured to prevent the flarelight is provided between the deflector 43 and the first optical lens 60a. The light-shielding wall 66 is provided in parallel to a YZ-plane asillustrated in FIG. 2. A seating surface configured to determine theposition in an optical axis direction of the first optical lens 60 a isprovided in a part of the light-shielding wall 66. In general, with thisconfiguration, a large scanning field angle can be provided so that thelight scanning apparatus 40 can be miniaturized.

As illustrated in FIG. 4A, by the presence of the light-shielding wall66 in the vicinity of the deflector 43, the current of air generatedwhen the deflector 43 rotates in a clockwise direction (indicated by anarrow E in FIG. 4A) in the case where the optical box 105 is viewed fromabove is strongly flowed out in directions indicated by arrows J. Thatis, in an apparatus of type in which a light beam is deflected in acounter direction as in the light scanning apparatus of the embodiment,hot current of air flows in a great amount toward the stepped portion68, and hence the efficiency for absorbing thermal deformation isimproved. Note that, the light-shielding wall 66 has a wall shape risingin the Z-axis direction, and hence the deformation of thelight-shielding wall 66 does not influence warpage and the like of theentire optical box 105, if any.

[Curved Portion or Bent Portion of Stepped Portion]

Further, as illustrated in FIG. 4B, the stepped portion 68B isconfigured so as to have a curved portion or a bent portion 68Bc(portion surrounded by a broken line in FIG. 4B) when the optical box105 is viewed from above in the rotation axis direction (Z-axisdirection) of the deflector 43. That is, as indicated by a two-headedbold arrow in FIG. 4B, the stepped portion 68B has an arc shape or asubstantially U-shape as viewed along the Z-axis direction. Note that,in FIG. 4B, the deflector 43 is omitted for ease of viewing of the shapeof the stepped portion 68B. In this way, in the periphery of thedeflector 43, the stepped portion 68B is provided in a portion in whichthere are no optical components such as an optical lens and hence astepped shape can be provided, so as to surround the deflector 43.Consequently, the deformation of the mirror seating surface and the likecan be further suppressed, and a change in irradiation position causedby increase in temperature can be reduced. Note that, the steppedportion 68A may also have a curved portion or a bent portion as viewedalong the Z-axis direction.

Note that, the stress acting in the directions indicated by the arrows Lillustrated in FIG. 3A acts radially around the deflector 43 on thebottom wall 105 a of the optical box 105. In other words, the stressacting in the directions indicated by the arrows L acts in all thedirections around the deflector 43 in the bottom wall 105 a, parallel tothe XY-plane, of the optical box 105. Therefore, ideally, it is desiredthat a stepped portion configured to absorb the stress acting in thedirections indicated by the arrows L be provided in the entire peripherysurrounding the deflector 43. However, actually, in order not toinfluence an optical path of a light beam from the light source unit 61to the deflector 43 and an optical path of a light beam from thedeflector 43 to the optical lens 60, there is a limit to the range inwhich the stepped portion can be provided in the periphery of thedeflector 43. In the embodiment, as an example, the stepped portions 68Aand 68B are configured as illustrated in FIG. 4B; however, it isappropriate to provide a stepped portion so that the stepped portionsurrounds the periphery of the deflector 43 as much as possible within arange not influencing an optical path of a light beam.

[Case Having Only One Step]

Now, an optical box in which a stepped portion having one large step isprovided as illustrated in FIG. 5 will be described. The stepped portionhaving only one step exhibits the effect of absorbing the deformation ofthe deflector bearing portion 69 and the portion immediately below theIC chip 67 to some extent so as not to influence the surrounding.However, as illustrated in FIG. 5, stagnant portions N3 of the hotcurrent of air generated by the deflector 43 occur, and hence the hotcurrent of air cannot smoothly flow throughout the light scanningapparatus. Therefore, in the stepped portion having only one step, thereis a risk in that the temperature difference may increase in theperiphery of the deflector 43 as well as the deflector bearing portion69 and the portion immediately below the IC chip 67, compared to anouter circumferential portion of the optical box 105, and a change inirradiation position of a light beam may increase. Therefore, thestepped portions 68A and 68B are set to have at least two steps as inthe embodiment.

[Effect of the Embodiment]

In order to confirm the effect of the embodiment, an amount ofdisplacement of the optical box 105 having the stepped portion 68 of aplurality of steps of the embodiment and an amount of displacement of anoptical box not having the stepped portion 68 of a plurality of stepswere measured and compared to each other. Further, the IC chip 67 andthe deflector bearing portion 69 were regarded as heat sources, andsimulation analysis was conducted considering heat transfer, radiation,and a flow current of air.

FIG. 6A is a graph showing an amount of displacement [μm (micrometer)]of the mirror seating surface which significantly influences a change inirradiation position of a light beam in the optical box 105. In FIG. 6A,the left bar represents an amount of displacement of an optical box(indicated by “OBLIQUE SLOPE”) having a conventional wall surface simplyprovided with a slope without the stepped portion 68, and the right barrepresents an amount of displacement of the optical box 105 (indicatedby “STEPPED SLOPE”) having the stepped portion 68 of the embodiment. Asshown in FIG. 6A, the amount of displacement of the conventional opticalbox is about 50 μm, whereas the amount of displacement of the opticalbox 105 of the embodiment is about 30 μm.

Further, FIG. 6B illustrates a thermal deformation analysis result ofthe conventional optical box not having a stepped portion, and FIG. 6Cillustrates a thermal deformation analysis result of the optical box 105having the stepped portion 68 of the embodiment. In the optical boxhaving a conventional configuration illustrated in FIG. 6B, thedeformation of a portion A′ enclosed by a broken line is particularlyconspicuous. In contrast, in the optical box 105 of the embodimentillustrated in FIG. 6C, it is understood that the deformation of aportion A enclosed by a broken line corresponding to the conventionalportion A′ is reduced compared to the conventional art. Further, in FIG.6B of the conventional art, the deformation of the light-shielding wall66 is larger than that of FIG. 6C of the embodiment, and hence it isunderstood that hot current of air which has occurred stagnates in thevicinity of the deflector 43.

Accordingly, when the conventional optical box not having a steppedportion is compared to the optical box 105 having the stepped portion 68of the embodiment, it is understood that the amount of displacement isreduced by about 40% in the embodiment, compared to the conventionalart. Further, similarly in the inspection of an actual product based onthe simulation, the reduction effect was confirmed.

As described above, according to the embodiment, the deformation of theoptical box can be reduced with a simple configuration.

Second Embodiment Configuration of Stepped Portion

FIG. 7 is a view illustrating the configuration of a stepped portion 168of a second embodiment. A stepped portion 168A and a stepped portion168B of the embodiment are configured so that the thickness of each wallforming the stepped portions 168A and 168B is non-uniform. In theembodiment, in the stepped portion 168B, a thickness of portions 168B1perpendicular to the bottom wall 105 a of the optical box 105 is thinnerthan a thickness of portions 168B2 parallel to the bottom wall 105 a ofthe optical box 105. Note that, the portion 168B1 corresponds to thefirst wall 68B1 of the first embodiment, and the portion 168B2corresponds to the second wall 68B2 of the first embodiment. Similarly,in the stepped portion 168A, a thickness of first walls 168A1corresponding to the first wall 68A1 of the first embodiment is thinnerthan a thickness of second walls 168A2 corresponding to the second wall68A2 of the first embodiment. With such a configuration, when a stressacting in directions indicated by arrows L is exerted on the steppedportions 168A and 168B, the stepped portions 168A and 168B are likely tobe deformed. Consequently, a change in irradiation position of a lightbeam caused by the deformation of the optical box 105 due to alocally-increased temperature can be reduced.

Further, a portion having a thickness difference in the stepped portion168 may be located at any position in the stepped portion 168A or thestepped portion 168B to exhibit the effects. For example, the thicknessof the portion 168B1 and the thickness of the portion 168B2 may bereversed. Specifically, the thickness of the portion 168B1 perpendicularto the bottom wall 105 a of the optical box 105 may be set to be thickerthan the thickness of the portion 168B2 parallel to the bottom wall 105a of the optical box 105. Even in such a case, the deformation of theoptical box 105 can be reduced. Note that, in the embodiment, adeformation stress generated in the bearing portion vicinity N1 of thedeflector bearing portion 69 and the portion N2 immediately below the ICchip 67, which are significantly influenced by heat, is parallel to thedirections indicated by the arrows L as illustrated in FIG. 3A, in otherwords, the bottom wall 105 a of the optical box 105. Therefore, asdescribed above, the thickness of the portion 168B1 perpendicular to thedirections indicated by the arrows L in which the stress acts is thin,and the thickness of the portion 168B2 parallel to the directionsindicated by the arrows L in which the stress acts is thick. With such aconfiguration, the effect of absorbing the deformation by the steppedportions 168A and 168B can be further produced.

As described above, according to the embodiment, the deformation of theoptical box can be reduced with a simple configuration.

Other Embodiment

In the foregoing embodiments, the case where the stepped portion 68(168) is formed so as to be integrated with the optical box 105 made ofa resin has been described. However, the stepped portion 68 (168) can beformed as a member separate from the optical box 105 and attached to theoptical box 105. In this case, a portion (hereinafter referred to as“attachment portion of the deflector 43”) corresponding to the bottomwall 105 a of the optical box 105 on which the deflector 43 is mountedis included in the separate member. As described above, by the heatgenerated from the deflector bearing portion 69 and the IC chip 67, astress acts radially on the bottom wall 105 a of the optical box 105around the deflector 43 so that the optical box 105 is deformed. Inorder for the stepped portion 68 (168) to absorb the deformation causedin the attachment portion of the deflector 43, it is necessary that theattachment portion of the deflector 43 and the stepped portion 68 (168)be formed integrally. Therefore, in the case where the stepped portion68 (168) is formed as a separate member, the attachment portion of thedeflector 43 which is influenced by radiation heat is included in theseparate member. That is, the attachment portion of the deflector 43 maybe formed of a member different from the optical box 105, and thestepped portion 68 (168) may be formed integrally with the attachmentportion.

As described above, also in the embodiment, the deformation of theoptical box can be reduced with a simple configuration.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2013-153623, filed Jul. 24, 2013, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A light scanning apparatus, comprising: a lightsource configured to emit a light beam; a deflector including a rotarypolygon mirror configured to deflect the light beam so that the lightbeam scans a photosensitive member, a motor configured to rotate therotary polygon mirror, a driving unit configured to drive the motor, anda circuit board on which the motor and the driving unit are mounted; aplurality of reflecting mirrors configured to reflect the light beamdeflected by the deflector to guide the light beam deflected by thedeflector onto the photosensitive member; and an optical box on whichthe light source is mounted, the optical box being configured to containthe deflector and the plurality of reflecting mirrors, wherein theoptical box includes an installation wall on which the deflector isinstalled and a support wall positioned on a side of the photosensitivemember with respect to the rotary polygon mirror, the support wall beingprovided with a support portion configured to support at least one ofthe plurality of reflecting mirrors, a stepped portion including aplurality of steps is formed between the installation wall and thesupport wall, and a back surface of the stepped portion has a shapefollowing a shape of the stepped portion inside the optical box.
 2. Alight scanning apparatus, comprising: a light source configured to emita light beam; a deflector including a rotary polygon mirror configuredto deflect the light beam so that the light beam scans a photosensitivemember, a motor configured to rotate the rotary polygon mirror, adriving unit configured to drive the motor, and a circuit board on whichthe motor and the driving unit are mounted; a plurality of reflectingmirrors configured to reflect the light beam deflected by the deflectorto guide the light beam deflected by the deflector onto thephotosensitive member; and an optical box on which the light source ismounted, the optical box being configured to contain the deflector andthe plurality of reflecting mirrors, wherein the optical box includes aninstallation wall on which the deflector is installed and a support wallpositioned on a side of the photosensitive member with respect to therotary polygon mirror, the support wall being provided with a supportportion configured to support at least one of the plurality ofreflecting mirrors, a stepped portion including a plurality of steps isformed between the installation wall and the support wall, and athickness of the stepped portion is smaller than a thickness of theinstallation wall.
 3. A light scanning apparatus, comprising: a lightsource configured to emit a light beam; a deflector including a rotarypolygon mirror configured to deflect the light beam so that the lightbeam scans a photosensitive member, a motor configured to rotate therotary polygon mirror, a driving unit configured to drive the motor, anda circuit board on which the motor and the driving unit are mounted; anoptical member configured to guide the light beam deflected by therotary polygon mirror onto the photosensitive member; and an optical boxon which the light source is mounted, the optical box being configuredto contain the deflector and the optical member, wherein the optical boxincludes an installation wall on which the deflector is installed, asupport wall positioned on a side of the photosensitive member withrespect to the rotary polygon mirror, the support wall being providedwith a support portion configured to support the optical member, and astepped portion including at least two steps in a vicinity of thedeflector.
 4. A light scanning apparatus according to claim 1, whereinthe plurality of steps of the stepped portion increase in height fromthe installation wall, from the installation wall toward the supportwall.
 5. A light scanning apparatus according to claim 1, wherein thestepped portion includes a curved portion or a bent portion.
 6. A lightscanning apparatus according to claim 1, wherein each of the pluralityof steps of the stepped portion includes a first wall perpendicular tothe installation wall and a second wall parallel to the installationwall, and the first wall has a thickness smaller than a thickness of thesecond wall.
 7. A light scanning apparatus according to claim 1, whereinthe optical box is made of a resin, and the stepped portion is formedintegrally with the optical box.
 8. A light scanning apparatus accordingto claim 1, further comprising a rib configured to reinforce the opticalbox, the rib being provided between the back surface of the steppedportion and an outer wall of the optical box on a side opposite to aside on which the light source is mounted with respect to the deflector.9. A light scanning apparatus according to claim 2, wherein theplurality of steps of the stepped portion increase in height from theinstallation wall, from the installation wall toward the support wall.10. A light scanning apparatus according to claim 2, wherein the steppedportion includes a curved portion or a bent portion.
 11. A lightscanning apparatus according to claim 2, wherein each of the pluralityof steps of the stepped portion includes a first wall perpendicular tothe installation wall and a second wall parallel to the installationwall, and the first wall has a thickness smaller than a thickness of thesecond wall.
 12. A light scanning apparatus according to claim 2,wherein the optical box is made of a resin, and the stepped portion isformed integrally with the optical box.
 13. A light scanning apparatusaccording to claim 3, wherein the plurality of steps of the steppedportion increase in height from the installation wall, from theinstallation wall toward the support wall.
 14. A light scanningapparatus according to claim 3, wherein the stepped portion includes acurved portion or a bent portion.
 15. A light scanning apparatusaccording to claim 3, wherein each of the plurality of steps of thestepped portion includes a first wall perpendicular to the installationwall and a second wall parallel to the installation wall, and the firstwall has a thickness smaller than a thickness of the second wall.
 16. Alight scanning apparatus according to claim 3, wherein the optical boxis made of a resin, and the stepped portion is formed integrally withthe optical box.
 17. A light scanning apparatus, comprising: a lightsource configured to emit a light beam; a deflector including a rotarypolygon mirror configured to deflect the light beam so that the lightbeam scans a photosensitive member, a motor configured to rotate therotary polygon mirror, a driving unit configured to drive the motor, anda circuit board on which the motor and the driving unit are mounted; aplurality of reflecting mirrors configured to reflect the light beamdeflected by the deflector to guide the light beam deflected by thedeflector onto the photosensitive member; and an optical box configuredto contain the deflector and the plurality of reflecting mirrors,wherein the optical box includes an installation wall on which thedeflector is installed and a support wall positioned on a side of thephotosensitive member with respect to the rotary polygon mirror, thesupport wall being provided with a support portion configured to supportat least one of the plurality of reflecting mirrors, and a waveformportion having a waveform in cross-section in a direction directed fromthe installation wall toward the support wall is formed between theinstallation wall and the support wall.
 18. A light scanning apparatusaccording to claim 17, wherein the waveform portion has a thicknesssmaller than a thickness of the installation wall.
 19. A light scanningapparatus according to claim 17, wherein the waveform portion has atriangular waveform.
 20. A light scanning apparatus according to claim17, further comprising a rib configured to reinforce the optical box,the rib being provided between the back surface of the waveform portionand an outer wall of the optical box on a side opposite to a side onwhich the light source is mounted with respect to the deflector.
 21. Alight scanning apparatus according to claim 17, wherein the waveformportion includes a curved portion or a bent portion.
 22. A lightscanning apparatus according to claim 17, wherein the optical box ismade of a resin, and the waveform portion is formed integrally with theoptical box.